The head, tail and lysis genes (late genes) of temperate phage P2 can be activated by two different mechanisms, one of which operates during normal infection, and another, which occurs when satellite phage P4 infects a P2-lysogenic cell. We are attempting to elucidate these two control mechanisms by isolating the P2 and P4 early gene products and the E. coli gene products which are needed for expression of P2 late genes in vitro. The head of satellite phage P4 is composed mostly of P2 head protein, but the P4 head is only one third the size of the P2 head. The satellite phage P4 genome has the potential to determine small head size. We have discovered two P4-specific late proteins, one of which is found in the P4 head (about 45 copies per head). The other P4 late protein is found in 100 S particles which may be preheads. The P4 late proteins may determine small head size. We can assay in vitro for head size determination, using a DNA packaging assay. For example, extracts from P4-infected, P2-lysogenic cells can package only the small DNA of P4, and not the large DNA from P2. BIBLIOGRAPHIC REFERENCES: Sunshine, M., Usher, D. and Calendar, R. Interaction of P2 bacteriophage with the dnaB gene of Escherichia coli. J. Virol. 16: 284-289, 1975.